Monitoring the Electrochemical Processes in the Lithium-Air Battery by Solid State NMR Spectroscopy.

Michal Leskes,Amy J Moore,Gillian R Goward,Clare P Grey

doi:10.1021/jp410429k

Abstract

A multi-nuclear solid-state NMR approach is employed to investigate the lithium–air battery, to monitor the evolution of the electrochemical products formed during cycling, and to gain insight into processes affecting capacity fading. While lithium peroxide is identified by 17O solid state NMR (ssNMR) as the predominant product in the first discharge in 1,2-dimethoxyethane (DME) based electrolytes, it reacts with the carbon cathode surface to form carbonate during the charging process. 13C ssNMR provides evidence for carbonate formation on the surface of the carbon cathode, the carbonate being removed at high charging voltages in the first cycle, but accumulating in later cycles. Small amounts of lithium hydroxide and formate are also detected in discharged cathodes and while the hydroxide formation is reversible, the formate persists and accumulates in the cathode upon further cycling. The results indicate that the rechargeability of the battery is limited by both the electrolyte and the carbon cathode stability. The utility of ssNMR spectroscopy in directly detecting product formation and decomposition within the battery is demonstrated, a necessary step in the assessment of new electrolytes, catalysts, and cathode materials for the development of a viable lithium–oxygen battery.

Highlights

The continued increase in global energy consumption and the shift toward electrification of transportation call for significant improvements in current lithium ion battery technology
Studies employing solution nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have identified that the instability of the lithium salt can reduce the cycle life of the cell due to side reactions that depend strongly on the combination of salt and solvent.[18−21] Much attention has been given to the porous carbon cathode in the cell
We have recently shown the strength of 17O solid state NMR (ssNMR) in investigating this system in part due to the unique NMR parameters of lithium peroxide,[33] demonstrating that the main discharge product in DME is lithium peroxide

Summary

INTRODUCTION

The continued increase in global energy consumption and the shift toward electrification of transportation call for significant improvements in current lithium ion battery technology. The spectrum of Li2CO3 was acquired on a Bruker 850 MHz Avance III spectrometer using a 4 mm double resonance probe using a single pulse excitation (nutation frequency of 42 kHz) with a relaxation delay of 15 s. Natural abundance 17O spectra of anhydrous lithium acetate (CH3O2Li) and lithium formate (HCO2Li) were acquired on a Bruker 900 MHz Avance II spectrometer, using a 4 mm double resonance probe with a rotor synchronized Hahn echo (nutation frequency 42 kHz). A natural abundance 17O spectrum of Li2O2 was acquired on a Bruker 850 MHz Avance III spectrometer using a static probe with a solid echo pulse excitation (62 kHz nutation frequency; 30 s relaxation delay). Spectra were fit using either SPINEVOLUTION40 spin dynamics simulation program or the line shape analysis tool in the Bruker software Topspin

RESULTS

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES